In transport means, such as trains, buses and airplanes, requirements in terms of connections to a broadband Internet service and requirements in terms of high-performance, low-cost and small-size antennas are growing. Currently, it is known to achieve a satellite link between a mobile terminal and a terrestrial station so as, for example, to ensure an Internet connection for passengers of a train or bus, by using a highly non-directional antenna operating in the L-band. The problem is that in the L-band there are very few frequencies available and that the transmission bitrate for the communications is therefore very low. To increase the bitrate, it is necessary to establish links with satellites operating in the Ku-band (10.5 GHz to 14.5 GHz) or the Ka-band (20 to 30 GHz) and to produce directional antennas. However, with a directional antenna, it is necessary to point the satellite continuously whatever the position of the vehicle. Moreover, in the Ku- or Ka-band, satellite-based transmissions may be masked by obstacles in the direction of pointing of a satellite. The obstacles may be vegetation or infrastructures such as buildings or bridges. It is therefore necessary, in order to increase the availability of the radiofrequency link with a satellite, to employ spatial diversity permitting the tracking of two satellites simultaneously. The two satellites having different directions of pointing, the simultaneous tracking of two satellites demands a dual-beam antenna with independent pointing, such as an electronic-scanning antenna, or two mechanical-pointing antennas.
In the case of an electronic-scanning antenna, to cover a territory such as Europe, the antenna must be able to ensure, in transmission and in reception, pointing in an angular domain lying between 0° and 360° in azimuth and between 20° and 60° on average in elevation.
These array antennas have the advantage of being plane and therefore of small size height-wise, however the angular domain to be covered being very large, in order to obtain good performance and avoid the appearance of array lobes in the antenna radiation pattern, it is necessary to use a beamforming array comprising a very large number of phase controls, this being prohibitive. For example, for an antenna in the Ku-band having an area of the order of 1 m2, the number of radiating elements of the antenna must be greater than 15000, this being crippling in terms of antenna cost and complexity for an application to transport means.
In the case of the use of a double antenna with mechanical pointing, each antenna fashions a beam by using an independent mechanical pointing system to point at the two satellites simultaneously. In this type of antenna, the pointing of each antenna in the direction of a satellite is achieved through a combination of two mechanical motions. A first mechanical motion is obtained by way of a rotating platform disposed in a plane XY and ensuring the orientation of the antenna azimuthally. A second motion in elevation is achieved through an ancillary device, for example a plane mirror, tied to the platform. Each antenna conventionally comprises a parabolic reflector and a radiating source illuminating the reflector. To decrease the size of the reflector and reduce the height of the antenna, its periphery is elliptical instead of circular. Typically, a simple antenna such as this, deployed at present on high-speed trains, exhibits a height of the order of 45 cm. Although this height is compatible with present-day trains, it is too large for future high-speed trains with two decks for which the height available for the installation of an antenna, between the roof of the train and the catenaries, is much smaller.
Likewise, for an application in the aeronautical field, the height of the antenna has an influence on the drag produced by the airplane as well as on fuel consumption. For example, the present-day reflector-type antennas installed on airplanes have a height of the order of 30 cm and give rise to extra fuel consumption equivalent to eight additional passengers.
Architectures exist which make it possible to reduce the height of the mechanical-pointing antenna. According to a first architecture, the antenna is composed of two parallel plates between which there circulate longitudinal components of current and of a one-dimensional array of transverse continuous grooves which couple and radiate the energy into space. The two plates and the array of grooves are mounted on two coplanar platens rotating mechanically independently of one another, the two rotation motions being superposed and carried out in the same plane of the platens. The orientation of the lower platen makes it possible to adjust the direction of pointing azimuthally, and the orientation of the upper platen makes it possible to obtain a variable inclination of the grooves and to thus modify the direction of pointing in elevation of the beam produced by the antenna. However, this antenna operating initially under linear polarization, it is necessary to supplement it with an additional orientable polarization grid mounted on the upper face of the antenna so as to control the plane of polarization of the antenna, thereby increasing the complexity of implementation and the height of the antenna which is then not plane.
According to a second architecture of plane antenna with reduced height, the antenna comprises several alternating planes of substrates and of metallic planes superposed one above the other. The antenna comprises a first lower metallic plane, and then a first substrate plane comprising several sources, the first substrate plane comprising a lateral end forming a parabolic surface on which the waves emitted by the sources are reflected. On top of the first substrate plane is a second metallic plane comprising slots for coupling the reflected waveplane, each coupling slot emerging in respective slot waveguides disposed side by side parallel to one another in one and the same second substrate plane. The guided waves are thereafter emitted in the form of a beam radiated through a plurality of radiating apertures made in a third upper metallic plane. Scanning and squinting of the beam in elevation, in a plane perpendicular to the plane of the antenna, is obtained by switching the various sources, but no modification of pointing in azimuth is possible. Moreover, this type of very compact antenna exhibits the drawback of requiring high-power switching means, this never being simple to achieve. Furthermore, the switching of the sources is discrete, thus not making it possible to obtain continuous pointing of the beam. Finally, this very compact antenna is fed by a unique power source, thereby making it necessary to use bulky power amplifiers which considerably increase the bulkiness of the antenna which becomes too large for an application to transport means.
To solve the problem of discrete pointing of this plane antenna, it has been proposed to use just a single source and to place the plane antenna on a rotating platform making it possible to adjust the pointing azimuthally, the platform comprising a mirror articulated on the platform whose angle of inclination with respect to the plane of the platform is variable by rotation. The plane wave emitted by the source illuminates the mirror which reflects this wave along a chosen direction of pointing, the angle of inclination of the mirror making it possible to adjust the angle of elevation of the beam emitted. This antenna is highly elliptical, the dimension of the mirror in its region articulated on the platform being much greater than the dimension of the mirror in its region inclined above the platform, thereby making it possible to reduce the height of the antenna to 20 or 30 cm, but this height still remains too large for an application to transport means.